Login at United.com and go to the My Itineraries section. If your mileage number is in your reservation you will be able to see your itinerary listed including your seat. Check out www.seatguru.com and compare the seat you have to what shows in My Itineraries.

As far as what your TA told you about upgrades... you are able to upgrade a C or D ticket to first class (which actually puts you in NF class..and this should also show in My Itineraries) with miles or a systemwide upgrade. There is no "automatic" upgrade that happens for free unless you get lucky at the airport with an operational upgrade. Did your TA use miles for this?